Vulcanization is an irreversible process during which a natural or synthetic rubber is reacted with sulfur or other vulcanizing agents, becoming less plastic and more elastic as a result. The usual vulcanizing agent is rhombic sulfur. Inorganic and organic accelerators are also added to rubber compounds to increase the rate and quality of curing. It has been found desirable to minimize the amount of rhombic sulfur in the unvulcanized composition, as rhombic sulfur is able to migrate to the surface of the rubber; this phenomenon is known as blooming. Blooming can reduce the adhesion of rubber to an adjacent layer of rubber or a substrate. For example, in pneumatic vehicle tires, adhesion between adjacent plies and adhesion of the rubber compound to textile, glass fiber, or metallic tire cords and belts is reduced by blooming, and separation of plies or other failure of the tire can result. Blooming can also mar the appearance of a molded rubber article.
One method to reduce blooming has been to use as a vulcanizing agent an allotropic form of sulfur with a reduced tendency to bloom. When rhombic or monoclinic allotropic forms of sulfur are heated to a temperature exceeding about 159.degree. C. and then quenched rapidly, they are at least partially converted to amorphous sulfur. Since the rhombic and monoclinic allotropes are soluble in carbon disulfide and amorphous sulfur is not, the latter is known as "insoluble sulfur". Insoluble sulfur is sold under the trademark CRYSTEX by Stauffer Chemical Company, Westport, Conn.
Insoluble sulfur is a better vulcanizing agent than rhombic sulfur, but is not a complete solution to the problem of blooming. Insoluble sulfur is relatively unstable, and slowly reverts to the rhombic form when stored at room temperature. Reversion of insoluble sulfur either before or after it is compounded into a rubber composition results in blooming. Also, rubber compositions are prone to scorching if processed at an unduly high temperature. Consequently, a vulcanizing agent with reduced blooming and greater scorch safety would be very beneficial.
Copolymers of sulfur and various unsaturated hydrocarbons, for example terpenoids, styrene, and dicyclopentadiene, are known. Reissued U.S. Pat. No. 31,575, issued to Ludwig et al. on May 1, 1984 (the original U.S. Pat. No. 4,290,816, issued Sept. 22, 1981), teaches as prior art an insoluble gel formed by reacting more than 5% dicyclopentadiene with sulfur at a temperature above 150.degree. C. (column 2, lines 4-13). Ludwig, et al. characterizes this gel as an undesired product and teaches away from the incorporation of more than 5% dicyclopentadiene in such a composition.
The following U.S. patents teach reaction products of sulfur, dicyclopentadiene and (in some cases) other reactants as vulcanization agents or extenders for rubber:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 3,563,962 Mirviss February 16, 1971 3,544,492 Taylor, et al. December 1, 1970 3,523,926 Mirviss August 11, 1970 3,264,239 Rosen, et al. August 2, 1966 2,806,843 Welch September 17, 1957 ______________________________________
While some of the above patents broadly suggest conducting the reaction of sulfur and dicyclopentadiene at a temperature exceeding 159 degrees Celsius, none of these references teach a two-step reaction or a thermosetting reaction product.
Three patents known to the present inventors teach reaction products of dicyclopentadiene, sulfur, and other components having other utilities than as vulcanizing agents. These patents are:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 4,391,969 McBee, et al. July 5, 1983 4,190,460 Cassar February 26, 1980 Re 31,575* ______________________________________ cited previously
The Cassar patent, Example 1 (especially lines 59-61), teaches 20% dicyclopentadiene and 80% sulfur reacted at 160.degree. C. to produce a brittle, glassy product. No reaction time is given, nor is the product described as thermosetting.
Numerous patents teach reaction products of styrene, sulfur, and (in some instances) other materials. Those teaching utility of the resulting composition as a rubber additive are:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 3,544,492* 3,259,598 Solomon July 5, 1966 3,231,546 Bertozzi January 25, 1966 2,989,513 Hendry, et al. June 20, 1961 ______________________________________ *cited previously
The patents teaching other utilities for reaction products of styrene and sulfur are:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 4,311,826 McBee, et al. January 19, 1982 4,188,297 Jayne, et al. February 12, 1980 4,147,640 Jayne, et al. April 3, 1979 4,119,550 Davis, et al. October 10, 1978 Re 31,575* ______________________________________ *cited previously
Terpenoids such as camphene, d-limonene, and dipentene (optically inactive limonene) and various unsaturated bicyclic hydrocarbons have also been combined with sulfur, and in some instances the reaction product has been suggested as a vulcanizing agent. The pertinent references are:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 4,311,826 McBee, et al. January 19, 1982 4,190,460* 4,147,640* 4,097,474 Askew, et al. June 27, 1978 3,544,492* Re 31,575* ______________________________________ *cited previously
The Jayne '640 patent cited above, at column 3, lines 40-46, teaches a two-stage heating process for reacting hydrogen sulfide, sulfur, and various olefins such as dicyclopentadiene, styrene, and limonene. The examples disclose that the reaction yields a liquid product useful as a lubrication additive.